If we're thinking Downton Abbey /Edwardian/WWI era, then popular posh names in England then were
Harold
Edward
Albert
Arthur
Alfred
Leopold
George
Ernest
Edgar
Frederick
Algernon
Reginald
John
Clarence
Horace
Victor
Gerald
Claude
Edwin
Lancelot
Theodore
Herbert
Augustus
Oswald
Scots had Scottish posh classics:
James
Alexander
Robert
Hugh
Donald
David
Roderick
Hector
nothing too dramatically Celtic - that was for radical/arty types and Scottish peasants
Also family surnames - if very grand and ancient, and ideally showing descent from the Norman conquerors - used as Christian names: think Baskerville, Fitzjames, Percival, Digby OR place names connected for centuries with the family,eg Wigmore,
Definitely not Jethro (=peasant), Sebastian (saint's name = only for aristocratic Catholics at that time, along with Aloysius etc)
But very much, as earlier poster said, nicknames: Bertie, Pongo, Wiggy etc